Renewable-energy sources, such as solar and wind, are being deployed in larger numbers than ever before, but these sources are intermittent and often unpredictable, and may provide energy only during the off-peak hours when power demand from consumers is low. These characteristics limit the extent to which utilities can rely upon them, and, as such, renewable energy sources currently comprise a small percentage of the primary power sources on the electrical grid. It has been suggested that the electrical grid could become destabilized if non-dispatchable renewable energy exceeds 20% of the energy-generation capacity without energy storage. However, many utilities are mandating renewable portfolios approaching this level of deployment. Thus, there is a pressing need for storage technologies to complement and enable renewable standards thereby providing renewable energy during peak demand. Furthermore, energy storage technologies are needed to provide demand response and other services to increase the reliability of the grid. Other than capacitors, however, there is no way to store electrical energy as such. Instead, if electricity is to be stored, it must first be converted to some other form of energy. There are some technologies that enable practical storage of energy at their current levels of deployment, but only a very small fraction of North American power plants employ such technology. To ensure that renewable energy succeeds in delivering reliable power to consumers, there needs to be cost effective and reliable storage at the grid scale.
Conventional rechargeable batteries offer a simple and efficient way to store electricity, but development to date has largely focused on transportation systems and smaller systems for portable power or intermittent backup power. These metrics relating to the size and volume are far less critical for grid storage than in portable or transportation applications. Batteries for large-scale grid storage require durability for large numbers of charge/discharge cycles as well as calendar life, high round-trip efficiency, an ability to respond rapidly to changes in load or input, and reasonable capital costs. Redox flow batteries or redox flow cells promise to meet many of these requirements.
A flow battery is a type of rechargeable battery where rechargeability is provided by two chemical components dissolved in liquids contained within the system and separated by a separator, for example, an ion-exchange membrane. A flow of ionic current occurs through the separator, while both liquids circulate in their own respective space. The energy storage capacity of the redox flow battery is fully decoupled from the available power, because the energy is related to the electrolyte volume, mass, and concentration (amount of liquid electrolyte) and the power to the number of cells included in the battery.
Currently, redox flow batteries are based on acidic electrolytes (e.g., hydrochloric and sulfuric acids). However, redox flow batteries based on acidic electrolytes suffer from considerable capacity fading due to species crossover and the occurrence of undesired secondary reactions during battery cycling (e.g. evolution of Cl2 gas). Acidic electrolytes are reactive (corrosive) to the cell components, including the separator or ion-exchange membrane, which translates into high operational and maintenance costs. Furthermore, manufacturing the electrolyte for such redox flow batteries requires electrolysis and/or other preparation steps that increase the cost of production.